Set Client Sample Coupled

Toggle whether the client's Block sample access follows live/bulk.

Entering MANUAL mode (False) is a pure flag flip — existing live/sample values are left untouched. Re-COUPLING (True) is blocked when any dataset has a live≠sample mismatch: the caller must resolve those first (409 sample_mismatch with the offending datasets/cuts). A clean re-couple needs no provider reconcile — every row already matches, so effective sample is unchanged.